Mastitis is characterized by physical, chemical and bacteriological
changes in the milk and pathological changes in the glandular tissue of
the udder and affects the quality and quantity of milk. The bacterial
contamination of milk from the affected cows render it unfit for human
consumption and provides a mechanism of spread of diseases like
tuberculosis, sore-throat, Q-fever, brucellosis, leptospirosis etc. and
has zoonotic importance. Somatic cell count (SCC) is a useful predictor
of intramammary infection (IMI) that includes leucocytes (75%) i.e.
neutrophils, macrophages, lymphocytes, erythrocytes and epithelial cells
(25%). Leucocytes increase in response to bacterial infection, tissue
injury and stress. Somatic cells are protective for the animal body and
fight infectious organisms. An elevated SCC in milk has a negative
influence on the quality of raw milk. Subclinical mastitis is always
related to low milk production, changes to milk consistency (density),
reduced possibility of adequate milk processing, low protein and high
risk for milk hygiene since it may even contain pathogenic organisms.
This review collects and collates relevant publications on the subject.
(Key Words : Mastitis, SCC, Factors, Management)

Mastitis, although an animal welfare problem, is a food safety
problem and is the biggest economic problem. Mastitis is characterized
by physical, chemical and bacteriological changes in the milk and
pathological changes in the glandular tissue of the udder (Sharma,
2007). It is also defined as inflammation of mammary gland parenchyma,
which is caused by bacteria and its toxins (Sharma et al., 2006). The
bacterial contamination of milk from affected cows render it unfit for
human consumption and provide a mechanism of spread of diseases like
tuberculosis, sore-throat, Q-fever, brucellosis, leptospirosis etc. and
has zoonotic importance (Sharif et al., 2009). The prevalence of
mastitis ranges from 29.34% to 78.54% (Sharma and Rai, 1977; Sharma and
Maiti, 2009) in cows and 66%-70.32% (Sharma et al., 2004; Sharma et al.,
2007) in buffaloes. Indirect methods such as the California Mastitis
Test (CMT), Sodium Lauryl Sulphate Test (SLST), Surf Field Mastitis Test
(SFMT) and White Side Test (WST) are available for the diagnosis of
mastitis under field conditions (as cow side test).

Somatic cells are indicators of both resistance and susceptibility
of cows to mastitis and can be used to monitor the level or occurrence
of subclinical mastitis in herds or individual cows. SCC is a useful
predictor of intramammary infection (IMI), and therefore, an important
component of milk in assessment of aspects of quality, hygiene and
mastitis control. Yet many producers fail to completely understand the
implications of SCC for udder health or how high SCC can affect milk
production and quality. Hence, this article contains detailed
explanation regarding SCC that will help researchers, academicians and
dairy farmers.

WHAT ARE SOMATIC CELLS?

Somatic cells are mainly milk-secreting epithelial cells that have
been shed from the lining of the gland and white blood cells
(leukocytes) that have entered the mammary gland in response to injury
or infection (Dairyman's digest, 2009). The milk somatic cells
include 75% leucocytes, i.e. neutrophils, macrophages, lymphocytes,
erythrocytes, and 25% epithelial cells. Erythrocytes can be found at
concentrations ranging from 0 to L51x[10.sup.6]/ml (Paape and Weinland,
1988). Studies identifying cell types in milk have shown that epithelial
cells or the cells which produce milk are infrequently found in udder
secretions, including the dry gland, at levels ranging from 0 to 7% of
the cell population (Lee et al., 1980). The epithelial cells of the
glands are normally shed and get renewed, however, during infection the
numbers increase. The white blood cells serve as a defense mechanism to
fight infection and assist in the repair of damaged tissue. During
inflammation (mastitis) the major increase in SCC is due to the influx
of neutrophils into the milk to fight infection and have been estimated
at over 90% (Miller and Paape, 1985; Harmon, 1994) and the measurement
of SCC in milk is known as a somatic cell count.

The normal composition of milk somatic cells varies with the type
of secretion or lactation cycle (Table 1). Normally, in milk from a
healthy mammary gland, the SCC is lower than 1x[10.sup.5] cells/ml,
while bacterial infection can cause it to increase to above
1x[10.sup.6]cells/ml (Bytyqi et al., 2010).

FUNCTION OF SOMATIC CELLS

Mastitis is caused by bacterial invasion into the udder. The small
numbers of somatic cells that are normally present in milk attempt to
resolve this intramammary infection immediately. The cellular presence
in milk is one of the important protective mechanisms of the mammary
gland and may be considered as a surveillance function in the uninfected
gland. Both bacteria and leukocytes in the infected quarters release
chemo-attractive products for leukocytes, especially neutrophils.

The neutrophil polymorphonuclear (PMN) leukocytes are the second
line of defense against mammary gland infection. PMN's are
phagocytic cells which engulf and kill bacteria. However, in bovines,
the phagocytic ability of PMN of milk can consume milk fat globules and
casein (Opdebeeck, 1982) leading to putrefaction of milk. An
inflammatory response is usually initiated when bacteria enter the
mammary gland through the teat canal and multiply in the milk. Although
bacterial toxins, enzymes and cell-wall components have a direct effect
on the function of the mammary epithelium, they it also stimulate the
production of numerous mediators of inflammation, mainly neutrophils
(Gallin et al., 1992), due to edema, vasodilation and increased vascular
permeability (Nonnccke and Harp, 1986).

Blood monocytes become macrophages in the tissues and are the major
cell type in milk during involution of the udder. During bacterial
pathogenesis, macrophages serve to facilitate either innate or acquired
immune responses. During lactation, the proportion of macrophages is
highest (68%) in the early post-partum period and lowest (21%) in late
lactation (Park et al., 1992). Similar to neutrophils, the non-specific
functions of macrophages are to phagocytise invading bacteria and
destroy them with proteases and reactive oxygen species (ROS) (Mullan et
al., 1985).

Lymphocytes are the only cells of the immune system that recognize
a variety of antigenic structures through membrane receptors, which
define their specificity, diversity and memory characters (Boyso et al.,
2007). T-lymphocytes and B-lymphocytes are two subsets of lymphocytes
that differ in function and protein products and play specific immune
functions (Harmon, 2001).

The mammary epithelial cells may play a protective role in
prevention of infection via ingestion and possible digestion of
phagocytosed microbes. The mammary epithelial cells are able to produce
a variety of inflammatory mediators such as cytokine, chemokines, host
defense peptides and arachidonic acid metabolites.

FACTORS AFFECTING SOMATIC CELL COUNT

There are plenty of factors that influence milk somatic cell count
at individual and herd level apart from intramammary infection. The
ability to correctly interpret somatic cell counts depends on an
understanding of the factors which may affect the number of somatic
cells.

Mammary gland infection level (Mastitis)

The most important factor affecting the somatic cell count of the
milk from an individual quarter depends upon the infection status of the
quarter (Dohoo and Meek, 1982). Sharma (2003) analyzed 2161 milk samples
from lactating cows and demonstrated that SCC [less than or equal
to]100,000 cells/ml could be considered as threshold or negative for the
California mastitis test (CMT) (Figure 1). The degree and nature of the
cellular response are likely to be proportional to the severity of the
infection (Figure 2). The average number of composite (cow) milk SCC
increases with an increase in the number of quarters infected (Meek et
al., 1980) and having a major influx of PMN into the milk (Craven and
Williams, 1985; Miller et al., 1990).

[FIGURE 1 OMITTED]

Stage of lactation

SCC increases with progressing lactation (late lactation)
regardless of whether the cow is infected or not (Dohoo and Meek, 1982).
SCC elevation has been linked with an animal's innate immune
response in preparation for calving and to enhance the mammary gland
defense mechanism at this critical calving time (Reichmuth, 1975).
During early and late lactation the percentage of neutrophils tends to
increase while the percentage of lymphocytes decreases (McDonald and
Anderson, 1981).

At parturition SCC are usually higher than one million per ml and
decreases to 100,000 cells/ml in the 7 to 10 days post-partum (Jensen
and Eberhart, 1981) (Table 2). The presence of high cell numbers has
also been reported in colostrum and appears due to an excessive
desquamation of epithelial cells in a small volume of milk in a gland
resuming functional activity after a dormant period (Schalm et al.,
1971).

[FIGURE 2 OMITTED]

Age/Breed

Various researchers have reported that SCC increases with
increasing age (Beckley and Johnson, 1966; Blackburn, 1966) (Table 3).
This increase is primarily due to an increased prevalence of infection
in older cows and is not due to any large increase due to age per se
(Reichmuth, 1975).

SCC variation has been noted between breeds of dairy animals. The
high-producing cattle breeds such as Brown Swiss (423.31x[10.sup.3]
cells/ml) and Black Holstein (310.36x [10.sup.3] cells/ml) have higher
presence of SCC/ml in milk. Different Indian breeds of cows with their
SCC's have been depicted by Singh (2002) and are shown in Table 4.

Parity/Season/Stress

The level of SCC has been reported to be influenced by parity
(Blackburn, 1966; Lindstrom et al., 1981). There is little change in SCC
of uninfected quarters as number of lactations increases (Sheldrake et
al., 1983) but SCC increases with advanced parities (Skrzypek et al.,
2004).

Somatic cell counts are generally lowest during the winter and
highest during the summer season (Khate and Yadav, 2010). During summer,
the growth and number of environmental bacteria is increased in the
bedding material of housed stock due to favorable temperature and
humidity (Harmon, 1994).

Free radicals are generally produced during stress due to milking
techniques, environmental and infectious organisms (teat injury). These
radicals are unstable and react quickly with other compounds in order to
capture the electron to gain stability (Smith et al., 1985).

Diurnal variation

In general, SCC that is lowest just before milking increases
rapidly on stripping, and may persist for up to 4 hours after milking
and then gradually declines. This difference in high and low SCC varies
from 4 to 70-fold for individual quarters (White and Rattray, 1965).
Studies have also shown that two consecutive milkings from the same cow
could fluctuate in SCC by 30%.

Day to day variation in cell counts has also been investigated and
revealed that SCC could fluctuate to more than 40% without any of the
circumstances described above.

There are many management factors that play a most important role
in the development of contagious disease like mastitis in dairy animals.
Amongst these, unhygienic conditions are more important in increasing
the chances of intramammary infection (IMI) and resulting in high SCC.
Other management factors pertain to the type of flooring, feeding, teat
dipping and milking techniques etc. Teat injuries and leakers commonly
develop because of stall and platform design raising the incidence of
mastitis and causing higher SCC. Using a post-milking teat dip appears
to predispose some very low SCC herds to more clinical mastitis-in
particular mastitis caused by E. coli. Recently, hygienic milking has
come into practice routinely to prevent the spread of Staph. aureus
inflicting contagious mastitis.

METHOD FOR MEASURING SOMATIC CELLS

More recently, automated devices for rapidly determining the SCC of
milk samples have become available. On-going development in counting
technology has resulted in the routine application of high capacity flow
cytometric counters with much improved performance in advanced milk
testing laboratories. The two most commonly-used devices are the Coulter
Milk Cell Counter, which counts particles as they flow through an
electric field, and the Fossomatic which stains cells with a fluorescent
dye and then counts the number of fluorescing particles. Both devices
are capable of rapid determination of the SCC in large numbers of
samples. Details of the procedures used by each device have been
published by various workers (Heeschen, 1975; Gonzalo et al., 2003) and
will not be discussed further in this paper. The direct microscopic
method is inexpensive and most commonly used in India (Sharma, 2003)
(Figure 3).

However, there is very little information on the specific
application of these methods in ewe milk (Gonzalo et al., 1993), which
has a higher content of total solids than cow milk. When evaluating
macrophages on a stained milk film, many will have a "foamy"
cytoplasm that could be analysed using the Fossomatic method (Gonzalo et
al., 2003).

[FIGURE 3 OMITTED]

SCC BASED INTERPRETATIONS

The two different methods may be used to calculate an "average
somatic cell count" when multiple samples have been taken. For
example, if the past three-month cell counts were 600,000, 400,000 and
500,000, the average would be calculated by arriving at a total and
dividing by 3, (1,500,000/3=500,000). This produces an "arithmetic
mean " or average.

A different method, used in Europe and other locations, is used to
calculate an average somatic cell count. It is termed the
"geometric mean". The geometric mean calculation always
produces a value somewhat less than the arithmetic mean for the same
data set. A single high count in a data set has a greater impact on the
arithmetic mean than the geometric mean and one very high value is not
as likely to trigger regulatory action using the geometric mean
procedure (Ingalls, 2001).

Research has established a straight-line relationship between milk
loss and the logarithm of the SCC. This value is referred to in Canada
as "Linear Score " (LS) and in the US as Somatic Cell Score
(SCS). Increase in linear score with the doubling of SCC has been
recorded by Ingalls (2001) as shown in Table 5.

All lactating cows have a low baseline SCC even if they do not have
an intramammary infection (IMI). When an infection is detected by the
immune system in a healthy cow, a rapid influx of leukocytes will
quickly raise the SCC far beyond the baseline level, usually to over a
million cells/ml.

In most developed dairy industries various regulatory limits has
been applied to milk for human consumption. The European Union
Directives (92/46CEE and 94/71 CEE) set a limit of 400,000 cells/ml for
SCC in raw buffalo milk, when the milk is used for products made with
raw milk. In US, the legal maximum somatic cell count for Grade A farm
bulk milk is 750,000 cells/ml, this limit is high compared to many
international standards. Much of Europe, New Zealand and Australia has a
limit of 400,000 cells/ml and Canada has a limit of 500,000 cells/ml of
raw milk. Milk SCC is a diagnostic figure for subclinical mastitis
(International Dairy Federation, 1999). Cow milk SCC of >200,000
cells/ml indicates mastitis (International Dairy Federation, 1997;
Hillerton, 1999). Recently, a line has been drawn for SCC that a level
below 100,000 cells/ml represents a healthy quarter. However, some
researchers consider a normal SCC to be up to 500,000 cells/ml. However,
it has been proposed that quarters having a cell count of 200000
cells/ml and whole cow milk cell count of 400,000 cells/ml to indicate
mastitis (Hillerton, 1999). Therefore, mastitis should be detected in a
reliable and timely fashion based on SCC values, otherwise subclinical
mastitis could develop into a clinical disease (Hallen Sandgren et al.,
2008).

SCC AND MASTITIS CAUSING ORGANISM

High SCC present in milk is the main indicator of mammary gland
infection, caused by specific and non-specific micro-organisms, which
cause contagious and environmental mastitis.

SCC increases of greater than 200,000 cells/ml have been observed
in cow milk as a result of bacterial infection. Various major or minor
pathogens display a moderate increase in somatic cells of approximately
50,000 cells/ml. The magnitude of SCC response to major pathogens varies
among cows, however, differentiation of types of pathogens seem
impossible based on SCC alone (Dohoo and Meek, 1982).

A study conducted by Boddie et al. (1987) showed the mean SCC of
quarters from unbred heifers infected with Staph. chromogenes, Staph.
hyicus, and Staph. aureus were 7.8, 8.5, and 9.2x[10.sup.6] cells/ml,
respectively. The mean SCC of uninfected quarters was 3.5x[10.sup.6]
cells/ml. The mean SCC of heifer secretions collected on the day of
freshening were 3.2x[10.sup.6] and 1.6x[10.sup.6] cells/ml for quarters
infected by staphylococci and uninfected quarters, respectively. The
mean SCC during the first 3 months of lactation in quarters infected
with Staph. chromogenes, Staph. hyicus, and Staph. aureus were 168, 193,
and 578x[10.sup.3] cells/ml, respectively, and SCC of uninfected
quarters was 39x[10.sup.3] cells/ml. However, SCC approached
20x[10.sup.6] cells/ml in quarters infected with Staph. aureus and over
13.6x[10.sup.6] cells/ml in those infected with coagulase-negative
staphylococci (CNS) and Streptococcus species. Such elevated SCC over a
long period of time suggests that affected quarters were in a state of
chronic inflammation, which could adversely affect development of
milk-producing tissues (Nickerson, 2009).

Sheldrake et al. (1983) compared lactation curves for SCC of
quarters free from clinical mastitis with lactation curves for SCC of
quarters with clinical Staph. aureus, coagulase-negative staphylococci
(CNS), and Corynebacterium bovis mastitis. He revealed that quarters
with clinical Staph. aureus mastitis showed a considerable increase in
SCC and quarters with known infection had higher SCC than quarters free
from clinical mastitis. Schepers et al. (1997) showed how different
pathogens caused changes or increases in quarter SCC. The largest
increase was found for Staph. aureus and the smallest for
Corynebacterium bovis.

Malinowski et al. (2006) carried out a study to determine the
relationship between SCC and mastitis etiological agents. They reported
that milk samples with SCC lower than 200,000 cells/ml were mostly
(59.6%) culture negative. Coagulase-negative staphylococci (CNS), Staph.
aureus and Streptococcus sp. were mostly noted in samples with 200,000
to 2,000,000 of SCC/ml. Samples having more than 2 million/ml of SCC
were infected mainly with CAMP-negative and CAMP-positive streptococci
and Gram negative bacilli. The highest SCC ([greater than or equal to]
10 million/ml) in foremilk samples were associated with intramammary
infections by Arcanobacterium pyogenes (95.5%), Streptococcus agalactiae
(57.6%) and Gram-negative organisms (46.5%). Very high SCC ([greater
than or equal to]5 million/ml) was connected with infections caused by
Prototheca sp. (64.5%), yeast-like fungi (60.2%) and Streptococcus sp.
(55.1%). Staph. aureus (76.2%), CNS (84.2%), Gram-positive bacilli
(72.4%) and Corynebacterium sp. (83.2%) caused an increase in SCC that
was smaller than 5 million/ml.

EFFECT OF SCC ON MILK QUALITY AND HUMAN HEALTH

Subclinical mastitis alters the composition of the milk in addition
to suppressing milk yield (Bramley, 1992; Harmon, 1994). Unlike milk
production loss, there is a direct relationship between SCC and milk
quality (Table 6). An elevated SCC in milk has a negative influence on
the quality of raw milk. Subclinical mastitis is always related to low
milk production (Bramley, 1992; Harmon, 1994), changes to milk
consistency (density), reduced possibility of adequate milk processing,
low protein and high risk for milk hygiene since it may even contain
pathogenic organisms. According to Harmon (1994), mastitis or elevated
SCC is associated with a decrease in lactose, a-lactalbumin, and fat in
milk because of reduced synthetic activity in the mammary tissue. The
largest negative consequences of the presence of SCC are related to
shorter shelf life and less sensory content or un-desirable
organo-leptic characteristics of the final product, due to enzymatic
activities of somatic cells (Topel, 2004). The higher levels of free
fatty acids in high cell count milk may produce a rancid flavor. Cheese
production from high cell-count milk has been reported to be lower than
from low cell-count milk (Everson, 1980). Decreasing SCC from 340,000 to
240,000 cells/ml increased cheese yield by 1% and decreasing SCC from
640,000 to 240,000 cells/ml increased cheese yield by 3.3%. The high
presence of SCC in milk affects the activity of yogurt fermentation
(Tamime and Robinson, 1999), and can even stop this process. Fernandes
et al. (2007) studied the effect of SCC in raw milk on the chemical and
physical properties of yogurt. He concluded that an increase in SCC
causes an increase in fatty acids in yogurt during the preservation
period and thus shortens the time of preservation of this product. The
reduced heat stability of high SCC milk causes flocculation during heat
treatment processes such as pasteurization and evaporation.

The relationship between raw milk somatic cell count and milk
components has been well documented (Ma et al., 2000; Schallibaum,
2001). The relationship between dairy cattle health and human health
warrants mention. The dairy industry strives to produce milk and dairy
foods that are safe and nutritious, and that are seen to be healthful
and wholesome. The greatest risk of high SCC milk to human health is in
the consumption of unpasteurized or improperly pasteurized milk (Oliver
et al., 2005). Viable pathogens and their toxins can be transferred from
the milk of infected quarters directly to humans. A large and diverse
group of human pathogens reside in the cow's environment including
Salmonella dublin, Campylobacter jejuni, and Listeria monocytogenes
(Oliver et al., 2005). These microbes are often pathogens or normal
flora of dairy cows. Evidence has been reported that Mycobacterium avium
subsp. paratuberculosis, associated with Johnes in cattle and isolated
from human patients with Crohn's disease, may survive some accepted
milk pasteurization procedures. Although the possible association
between shedding of the Mycobacterium avium subsp. paratuberculosis in
milk and subsequent survival after pasteurization is compelling, the
rate of shedding is low in infected cows and not related to an increase
in SCC (Stabel, 2005). Surveys indicate that dairy producers and their
families drinking milk produced on their own farms are among the
demographic groups at greatest risk to food-borne diseases due to
consumption of unpasteurized milk. Proper pasteurization of milk is very
effective in preventing the transfer of viable pathogens from milk of
infected mammary glands to humans. Pasteurization reduces the number of
viable microorganisms, but often does not negate the effects of toxins
produced by mastitis pathogens.

There are a number of diseases of dairy cattle and pathogens
transmissible from dairy cattle that are zoonotic. Direct ingestion of
bovine neutrophils has been reported to cause health problems. As the
SCC increases, the percentage of cells, particularly neutrophils,
increases. Therefore, the potential health risk of consuming milk with
an elevated SCC would depend largely on the human health concerns of
ingesting bovine neutrophils. Although the ingestion of large numbers of
bovine neutrophils in milk may be objectionable, direct negative effects
on the safety of humans have not been documented as a result of
consuming dairy products made with milk having high SCC.

REDUCTION AND MANAGEMENT OF HIGH SCC

Bacterial invasion occurs mostly during the dry period,
particularly during late gestation, and leads to glandular damage in
parenchymatous tissue. The glandular tissue damage leads to increased
SCC and reduced milk production. To reduce the occurrence of mastitis
and control SCC, prevention strategies should be followed during the dry
period. A few of the latest prevention strategies that have been
recommended, which cover animal and environment hygiene, have included
the use of teat sealants, teat antiseptics, pre-calving milking, control
of insects and segregation of pregnant heifers from older cows etc.

Furthermore, the standard mastitis control program decreases the
prevalence of intramammary infections with contagious pathogens
(Hillerton et al., 1995), but it has been rated relatively low in
success in prevention of clinical mastitis from environmental pathogens
(Lam et al., 1997b; Barkema et al., 1999). Recommendations to control
both contagious and environmental pathogens have been combined in a new
ten-point mastitis control program, issued by the National Mastitis
Council (2001). In this program, lactation-average somatic cell count
(SCC) has been generally used to control mastitis.

The currently used primary parameters to analyse the herd situation
in the mastitis control program are: i) bulk milk somatic cell count,
ii) percentage of cows with SCC >250,000 cells/ml per test-day, iii)
percentage of cows with new infections and iv) culling rate because of
mastitis.

Nutritional supplementation with vitamins and minerals enhances the
immunity of the animal and therefore decreases SCC numbers. Impact of
such supplements have been already demonstrated with the use of vitamin
E (at 500 IU/animal/d) and selenium (at 6 mg/animal/d) alone or in
combination for two months during early lactation to control the
intramammary infection and to manage SCC (Sharma and Maiti, 2005)
(Figure 4). Such dietary supplementation of vitamin E and selenium in
combination showed reduction in the SCC from 29.39x[10.sup.5] to
8.28x[10.sup.5] cells/ml of milk.

[FIGURE 4 OMITTED]

Pre-calving antibiotic treatment was also found by Bastan et al.
(2010) to be quite effective in reducing individual quarter SCC. Sharma
et al. (2007) and Sharma (2008) have also reported a drastic decrease in
SCC during clinical mastitis after treatment with enrofloxacin
antibiotic.

[FIGURE 5 OMITTED]

Several studies in the past have shown a positive, unfavorable
genetic correlation between milk yield and clinical mastitis (Shook,
1993; Rogers et al., 1998); this implies that genetic improvement for
milk yield has been accompanied by increased genetic susceptibility to
mastitis. Therefore, it is important to place some selection emphasis on
udder health traits to offset the undesirable genetic trend towards
mastitis susceptibility that results from selection for increased milk
yield. Furthermore, there is also an utmost need for establishment of
selection of an animal's treatment for mastitis based on
hematopoietic stem cell differentiation into innate immune cells and
control of stem cell differentiation under the animal disease
environment for discovery of a self-cure mechanism.